In recent years, research and development of a thin film such as PZT or SBT, and a ferroelectric capacitor and a ferroelectric memory device using such a thin film have been extensively conducted. The structure of the ferroelectric memory device is roughly divided into 1T, 1T1C, 2T2C, and simple matrix. Since the 1T ferroelectric memory device has retention time (data retention time) as short as one month due to occurrence of an internal electric field in the capacitor because of its structure, it is considered to be impossible to secure a 10-year guarantee generally required for semiconductors. The 1T1C and 2T2C ferroelectric memory devices have almost the same configuration as that of a DRAM, and include a select transistor. Therefore, the DRAM manufacturing technology can be utilized. Moreover, since the 1T1C and 2T2C ferroelectric memory devices realize a write speed equal to that of an SRAM, small capacity products with a capacity of 256 kbits or less have been produced on a commercial basis.
As the ferroelectric material, Pb(Zr,Ti)O3 (PZT) has been mainly used. PZT having a composition in or near a mixed region of a rhombohedral crystal and a tetragonal crystal, in which the Zr/Ti ratio is 52/48 or 40/60, is used after doping with an element such as La, Sr, or Ca. The reason why this region is used after doping is to secure reliability indispensable for a memory device.
PZT, which has been widely applied to a ferroelectric memory, is a solid solution of PbZrO3 and PbTiO3. In the case where PZT contains Zr at a ratio greater than Zr:Ti=52/48, PZT shows a narrow hysteresis shape. In the case where PZT contains a greater amount of Ti, PZT shows a hysteresis shape with excellent squareness.
The hysteresis shape immediately after writing data is better in the Ti-rich tetragonal region. The Ti-rich tetragonal region is suitable for memory applications if only the hysteresis shape is taken into consideration. However, PZT in the tetragonal region has not been put into commercial use as a ferroelectric memory device since reliability cannot be secured.
Among a number of reliability tests, a reliability test called a static imprint test is the most rigorous test. In this test, a ferroelectric memory in which data “1” or “0” is written, that is, a ferroelectric thin film polarized at “+” or “−” is allowed to stand at a predetermined temperature (85° C. or 150° C., for example) for a predetermined time (100 or 1000 hours, for example), and whether or not the originally written data can be read is tested.
As described above, the hysteresis shape immediately after writing data is better in the Ti-rich tetragonal region. However, the Ti-rich tetragonal region means that most of the crystal constituent elements consist of Pb and Ti.
Pb has a high vapor pressure, and produces PbO vapor at a low temperature of about 100° C. as known from the Ellingham diagram. Moreover, Pb has the smallest bond energy with oxygen (38.8 kcal/mol), and tends to cause Pb deficiency to occur in the PZT crystal. Ti has a bond energy with oxygen of 73 kcal/mol, which is about twice the Pb—O bond energy. However, since Ti has the smallest atomic weight (47.88) among the constituent elements of PZT, which is about half the atomic weight of Zr (91.224) which is also a B site constituent element, it is most likely that Ti is scattered during oscillating bombardment which occurs during the heat treatment in the static imprint test, whereby Ti deficiency tends to occur in the PZT crystal. These defects result in space charge polarization, and cause imprint characteristics to deteriorate.
Moreover, O deficiency occurs from the charge neutrality principle, whereby Schottky defects occur due to the ionic crystal structure. This causes leakage current characteristics to deteriorate, whereby reliability cannot be secured.
A reduction of the device size and the thickness of the ferroelectric thin film has progressed accompanying an increase in the degree of integration of the ferroelectric memory and the necessity of low voltage drive. Therefore, in the case of using PZT as the ferroelectric material, it is impossible to utilize the Zr-rich composition used for a small capacity memory. This makes it necessary to use PZT having a Ti-rich composition.
Specifically, since the relative dielectric constant is increased due to a reduction of the film thickness, the hysteresis shape becomes narrower. Zr-rich PZT has not posed problems relating to reliability such as imprint characteristics in practical application. However, if the hysteresis shape becomes further narrowed, deterioration of imprint characteristics will come to the surface. Therefore, in order to make the hysteresis shape closer to the hysteresis shape used for a small capacity memory by decreasing the relative dielectric constant, PZT having a Ti-rich composition must be used. This causes the above-described problems to occur. Therefore, it is impossible to increase the integration of the ferroelectric memory unless the problems of Ti-rich PZT are solved.
The simple matrix ferroelectric memory device has a cell size smaller than that of the 1T1C and 2T2C ferroelectric memory devices, and enables multilayering of the capacitors. Therefore, an increase in the degree of integration and a reduction of cost are expected. A conventional simple matrix ferroelectric memory device is disclosed in Japanese Patent Application Laid-open No. 9-116107, for example. Japanese Patent Application Laid-open No. 9-116107 discloses a drive method in which a voltage of one-third a write voltage is applied to unselected memory cells when writing data into the memory cell. However, this technology does not describe the hysteresis loop of the ferroelectric capacitor necessary for the operation in detail. The present inventors have advanced development and found that a hysteresis loop with excellent squareness is indispensable to obtain a simple matrix ferroelectric memory device which can be operated in practice. As a ferroelectric material which can deal with such a requirement, Ti-rich tetragonal PZT may be considered as a candidate. However, the most important subject is to secure reliability in the same manner as the 1T1C and 2T2C ferroelectric memories.
A ferroelectric thin film used for a ferroelectric memory is generally used in a state in which the polarization axis of the ferroelectric is aligned in the direction of an applied electric field.
In PZT, the Zr/Ti ratio of 52/48 is called a phase boundary, which is a mixed region of a rhombohedral crystal and a tetragonal crystal. If Zr exceeds 52, the crystal structure becomes rhombohedral. If Ti exceeds 48, the crystal structure becomes tetragonal.
In rhombohedral PZT, the polarization axis exists along the <001> axis. In tetragonal PZT, the polarization axis exists along the <111> axis. Therefore, in the case of using a PZT thin film for a ferroelectric memory, the PZT thin film is generally used in a state in which the orientation is aligned with the polarization axis direction, as described in 49th Japan Society of Applied Physics and Related Societies Meeting Preliminary Report 27a-ZA-6.
However, domains which are sources of ferroelectricity are present in the ferroelectric in addition to the crystal orientation. The domains include a 180° domain and a 90° domain.
In the case where the polarization axis is aligned with the crystal orientation axis, the 180° domain parallel to the applied electric field contributes to polarization, but the 90° domain does not contribute to polarization.
If an ideal ferroelectric capacitor is formed, since the 90° domain does not contribute to polarization, a serious problem does not occur even if the 90° domain exists. However, the contribution rate of the entire PZT thin film to polarization is reduced by an amount for the existence of the 90° domain.
In the actual ferroelectric capacitor, the uppermost surface of the electrode is not completely flat and has unevenness, and a crystal is grown in an inclined state in most cases. In this case, the 90° domain does not become completely perpendicular to the applied electric field, and is at an angle to the applied electric field to some extent. In this case, the 90° domain contributes to polarization. However, since the polarization axis exists in the direction approximately at right angles to the applied electric field, a considerably large electric field is necessary to cause the 90° domain to contribute as the polarization in comparison with the 180° domain. Specifically, it becomes difficult to use the ferroelectric capacitor at a low voltage.